For elongated objects, the notion of ''ridges'' is a natural tool. A ridge descriptor computed from a grey-level image can be seen as a generalization of a [[Medial_axis|medial axis]]. From a practical viewpoint, a ridge can be thought of as a one-dimensional curve that represents an axis of symmetry, and in addition has an attribute of local ridge width associated with each ridge point. Unfortunately, however, it is algorithmically harder to extract ridge features from general classes of grey-level images than edge-, corner- or blob features. Nevertheless, ridge descriptors are frequently used for road extraction in aerial images and for extracting blood vessels in medical images—see [[ridge detection]].
